Can I buy a .40 and then put a 6" 10mm aftermarket barrel on it or is the frame/strength/build such that I really need to buy the pistol as a 10mm and then buy aftermarket barrels in both .40 and 10mm? (Obviously, I'm hoping for the former.)

Sorry, doesn't work that way. The length of the bullet wouldn't let you insert the mag in a Glock 22 for instance. Plus, the slide is thicker/stronger for the 10mm round.